As shown in FIG. 5, an electrooptic crystal 1 is brought close to an object 3 of measurement, with a transparent electrode 2 bonded to the crystal 1 and grounded. Assuming that the object 3 has a predetermined surface potential, lines of electric force from the object 3 terminate at the electrode 2 through the electrooptic crystal 1 and a predetermined electric field is therefore applied to the electrooptic crystal 1, resulting in a change in the optical anisotropy. Since the change in the optical anisotropy is proportional to the magnitude of the electric field, if polarized light 4, for example, is applied, the polarization of the reflected or transmitted light changes. The retardation is expressed by ##EQU1## where .DELTA..phi. is the retardation, n is the refractive index, .gamma. is the electrooptic constant, E is the electric field, l is the optical path length and .lambda. is the wavelength. The surface potential of the object Vs is given by ##EQU2## where (d.sub.1, E.epsilon..sub.1), (d.sub.2, .epsilon..sub.2) and (d.sub.3, .epsilon..sub.3) are the thicknesses and dielectric constants of the electrooptic crystal, the air gap and the object of measurement, respectively. If the object is in contact with the electrooptic crystal (i.e., d.sub.2 =0) and the object is metallic (i.e., d.sub.3 =0), the following relationship holds: EQU Vs=Ed.sub.1 ( 3)
In other words, El in the expression (1) corresponds to Vs, and in the case of a reflecting system, l=2d.sub.1, whereas, in the case of a transmitting system, l=d.sub.1. Thus, the surface potential Vs of the object can be determined by obtaining .DELTA..phi..
This type of optical potential sensor has heretofore been produced by forming a transparent electrode (ITO) 2 on a glass substrate 6 by sputtering or the like, as shown in FIG. 6, bonding LiNbO.sub.3 1 to the surface of the transparent electrode 2 by means of an adhesive 7, grinding the LiNbO.sub.3 1 to a necessary thickness, and then forming a dielectric reflecting film 8 thereon by use of thin-film technology. Thus, the conventional optical potential sensor has a complicated structure. In addition, since the bonding step and the grinding step are involved, the production process is complicated and the processing characteristics are inferior. Further, the surface accuracy is low because of the thickness unevenness that is caused by the bonding and the grinding, which results in low measuring accuracy. In addition, since LiNbO.sub.3 has optical properties which are different in all of the three directions, it is difficult to incline the Z-axis of the crystal exactly at 55.degree. with respect to the direction of incidence of light. Further, LiNbO.sub.3 involves the problem of photo-deterioration.
It is an object of the present invention to enable potential measurement using an electrooptic crystal which is easy to produce.
It is another object of the present invention to enable highly accurate potential measurement without photo-deterioration by use of an electrooptic crystal.